Circuit Description

Intake air temperature sensor is built into the mass airflow sensor and senses the intake air temperature. A thermistor built in the sensor changes the resistance value according to the intake air temperature. The lower the intake air temperature becomes, the greater the thermistor resistance value, and the higher the intake air temperature becomes, the lower the thermistor resistance value. The intake air temperature sensor is connected to the ECM. The 5 volt power source voltage in the ECM is applied to the intake air temperature sensor from terminal THA via resistor "R". Resistor "R" and the intake air temperature sensor are connected in series. When the resistance value of the intake air temperature sensor changes in accordance with changes in the intake air temperature, the potential at terminal THA also changes. Based on this signal, the ECM increases the fuel injection volume to improve driveability during cold engine operation. See Fig 1 and Fig 2 . See INTAKE AIR TEMPERATURE RESISTANCE  table. Possible causes are:

• Open or short in IAT sensor circuit.
• Faulty IAT sensor.
• Faulty ECM.

Fig 1: Identifying Intake Air Temperature Acceptable Range

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Fig 2: Identifying Mass Airflow Sensor Circuits

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